linux-kernel - Re: Linux 2.6.29

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Message-ID: <49D0F399.5010407@rtr.ca>
Date:	Mon, 30 Mar 2009 12:30:17 -0400
From:	Mark Lord <lkml@....ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Theodore Tso <tytso@....edu>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Jeff Garzik <jeff@...zik.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Linus Torvalds wrote:
> 
> On Mon, 30 Mar 2009, Ric Wheeler wrote:
>> A modern S-ATA drive has up to 32MB of write cache. If you lose power or
>> suffer a sudden reboot (that can reset the bus at least), I am pretty sure
>> that your above assumption is simply not true.
> 
> At least traditionally, it's worth to note that 32MB of on-disk cache is 
> not the same as 32MB of kernel write cache.
> 
> The drive caches tend to be more like track caches - you tend to have a 
> few large cache entries (segments), not something like a sector cache. And 
> I seriously doubt the disk will let you fill them up with writes: it 
> likely has things like the sector remapping tables in those caches too.
..

I spent an entire day recently, trying to see if I could significantly fill
up the 32MB cache on a 750GB Hitach SATA drive here.

With deliberate/random write patterns, big and small, near and far,
I could not fill the drive with anything approaching a full second
of latent write-cache flush time.

Not even close.  Which is a pity, because I really wanted to do some testing
related to a deep write cache.  But it just wouldn't happen.

I tried this again on a 16MB cache of a Seagate drive, no difference.

Bummer.  :)
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